Bitmap: clearing pixelref keeps rowbytes

Change-Id: Ibf8b69adcb4e9df597079a8d9ada75b4a31194e6
Reviewed-on: https://skia-review.googlesource.com/94040
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
2 files changed